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CORRECTIONS AND ADDITIONS
BY THE AUTHOR.
LINE 12. A far more important result in reference to the density of the earth than that obtained by Baily (1842) and Reich (1847-1850) has been brought out by Airy's experiments with the pendulum, conducted with such exemplary care in the Mines of Harton, in the year 1854. According to these experiments, the density is 6.566, with a probable error of 0·182 (Airy in the Philos. Transact. for 1856, p. 342). A slight modification of this numerical value, made by Professor Stokes on account of the effect of the rotation and ellipticity of the earth, gives the density for Harton, which lies at 54° 48' north latitude, at 6:565, and for the Equator at 6:489. PAGE 76.
LINE 10. Arago has left behind him a treasury of magnetical observations (upwards of 52,600 in number) carried on from 1818 to 1835, which have been carefully edited by M. Fedor Thoman, and published in the Euvres complètes de François Arago (t. iv, p. 498). In these observations, for the series of years from 1821 to 1830, General Sabine has discovered the most complete confirmation of the decennial period of magnetic declination, and its correspondence with the same period, in the alternate frequency and rarity of the solar spots (Meteorological Essays, London, 1855, p. 350). So early as the year 1850, when Schwabe published at Dessau his notices of the periodical return of the solar spots (Cosmos, vol. iv, p. 397), two years before Sabine first showed the decennial period of magnetic declination to be dependent on the solar spots (in March, 1852, Phil. Tr. for 1852, p. i, pp. 116—121 ; Cosmos, vol. v, p. 76, note), the latter had already discovered the important result, that the sun operates on the earth's magnetism by the magnetic power proper to its mass. He had discovered (Phil. Tr. for 1850, p. i, p. 216, Cosmos, vol. v, p. 140), thụt the magnetic intensity is greatest, and that the needle approaches nearest to the vertical direction, when the earth is nearest to the sun. The knowledge such a magnetical operation of the central body of our planetary system, not by its heatproducing quality, but by its own magnetic power, as well as by changes in the Photosphere (the size and frequency of funnel-shaped openings), gives a higher cosmical interest to the study of the earth's magnetism and to the numerous magnetic observatories (Cosmos, vol. 1, p. 184; vol. v, p. 73) now planted over Russia and Northern Asia, since the resolutions of 1829, and over the colonies of Great Britain since 1840— 1850. (Sabine, in the Proceedings of the Roy. Soc. vol. viii, No. 25, p. 400; aud in the Phil. Trans. for 1856, p. 362).
CORRECTIONS AND ADDITIONS.
LINE 9. Though the nearness of the moon in comparison with the sun does not seem to compensate the smallness of her mass, yet the already well ascertained alteration of the magnetic declination in the course of a lunar day, the lunar-diurnal magnetic variation (Sabine, in the Report to the Brit. Assoc. at Liverpool, 1854, p. 11, and for Hobart-town in the Phil. Tr. for 1857, Art. i, p. 6), stimulates to a persevering observation of the magnetic influence of the earth's satellite. Kreil has the great merit of having pursued this occupation with great care, from 1839 to 1852, (see his treatise Ueber den Einfluss des Mondes auf die horizontale Component der Magnetischen Erdkraft
, in the Deukschriften der Wiener Akademie der Wiss. Mathem. Naturwiss. Classe, vol. V, 1853, p. 45, and Phil. Trans. for 1856, Art. xxii). His observations, which were conducted for the space of many years, both at Milan and Prague, having given support to the opinion that both the moon and the solar spots occasioned a decennial period of declination, led General Sabine to undertake a very important work. He found that the exclusive influence of the sun on a decennial period, previously examined in relation to Toronto in Canada, by the employment of a peculiar and very exact form of calculation, may be recognised in all the three elements of terrestrial magnetism (Phil. Trans. for 1856, p. 361), as shown by the abundant testimony of hourly observations carried on for a course of eight years at Hobart Town, from January 1841 to December 1848. Thus both hemispheres furnished the same result as to the operation of the sun, as well as the certainty “that the lunar-diurnal variation corresponding to different years shows no conformity to the inequality manifested in those of the solar-diurnal variation. The earth's inductive action, reflected from the moon, must be of a very little amount.” (Sabine, in the Phil. Tr. for 1857, Art. i, p. 7, and in the Proceedings of the Royal Soc. vol. viii, No. 20, p. 404). The magnetic portion of this volume having been printed almost three years ago, it seemed especially necessary, with reference to a subject which has so long been a favourite one with me, that I should supply what was wanting by some additional remarks.
SPECIAL RESULTS OF OBSERVATION IN THE DOMAIN
OF TELLURIC PHENOMENA.
In a work embracing so wide a field as the Cosmos, which aims at combining perspicuous comprehensibility with general clearness, the composition and co-ordination of the whole are perhaps of greater importance than copiousness of detail. This mode of treating the subject becomes the more desirable because, in the Book of Nature, the generalization of views, both in reference to the objectivity of external phenomena and the reflection of the aspects of nature upon the imagination and feelings of man, must be carefully separated from the enumeration of individual results. The first two volumes of the Cosmos were devoted to this kind of generalization, in which the contemplation of the Universe was considered as one great natural whole, while at the same time care was taken to show how, in the most widely remote zones, mankind had, in the course of ages, gradually striven to discover the mutual actions of natural forces. Although a great accumulation of phenomena may tend to demonstrate their causal connection, a General Picture of Nature can only produce fresh and vivid impressions when, bounded by narrow limits, its perspicuity is not sacrificed to an excessive aggregation of crowded facts.
As in a collection of graphical illustrations of the surface and of the inner structure of the earth's crust, general maps precede those of a special character, it has seemed to me that in a physical description of the Universe it would be most appropriate, and most in accordance with the plan of the present work, if, to the consideration of the entire Universe from general and higher points of view, I were to append in the latter volumes those special results of observation upon
which the present condition of our knowledge is more particularly based. These volumes of my work, must, therefore, in accordance with a remark already made (Cosmos, vol. üi, pp. 2–6), be considered merely as an expansion and more careful exposition of the General Picture of Nature (Cosmos, vol. i, pp. 62—369), and as the uranological or sidereal sphere of the Cosmos was exclusively treated of in the two last volumes, the present volume will be devoted to the consideration of the telluric sphere. In this manner the ancient, simple, and natural separation of celestial and terrestrial objects has been preserved, which we find by the earliest evidences of human knowledge to have prevailed among all nations.
As in the realms of space, a transition to our own planetary system from the region of the fixed stars, illumined by innumerable suns, whether they be isolated or circling round one another, or whether they be mere masses of remote nebulæ, is indeed to descend from the great and the universal to the relatively small and special ; so does the field of our contemplation become infinitely more contracted when we pass from the collective solar system, which is so rich in varied forms, to our own terrestrial spheroid, circling round the sun. The distance of even the nearest fixed star, a Centauri, is 263 times greater than the diameter of our solar system, reckoned to the aphelion distance of the comet of 1680; and yet this aphelion is 853 times further from the sun than our earth (Cosmos, vol. iv, p. 546). These numbers, reckoning the parallax of a Centauri at 0."9187, determine approximately both the distance of a near region of the starry heavens from the supposed extreme solar system and the distance of those limits from the earth's place.
Uranology, which embraces the consideration of all that fills the remote realms of space, still maintains the character it anciently bore, of impressing the imagination most deeply and powerfully by the incomprehensibility of the relations of space and numbers which it embraces ; by the known order and regularity of the motions of the heavenly bodies ; and by the admiration which is naturally yielded to the results of observation and intellectual investigation. This consciousness of regularity and periodicity was so early impressed upon the human mind that it was often reflected in those forms of speech, which refer to the ordained course of the celestial bodies. The known laws which rule the celestial sphere excite perhaps the greatest admiration by their simplicity, based as they solely are, upon the mass and distribution of accumulated ponderable matter and upon its forces of attraction. The impression of the sublime, when it arises from that which is immeasurable and physically great, passes almost unconsciously to ourselves beyond the mysterious boundary which connects the metaphysical with the physical, and leads us into another and higher sphere of ideas. The image of the immeasurable, the boundless, and the eternal, is associated with a power which excites within us a more earnest and solemn tone of feeling, and which, like the impression of all that is spiritually great and morally exalted, is not devoid of emotion.
The effect which the aspect of extraordinary celestial phenomena so generally and simultaneously exerts upon entire masses of people, bears witness to the influence of such an association of feelings. The impression produced in excitable minds by the mere aspect of the starry vault of heaven is increased by profounder knowledge, and by the use of those means which man has invented to augment his powers of vision, and at the same time enlarge the horizon of his observation. A certain impression of peace and calmness blends with the impression of the incomprehensible in the universe, and is awakened by the mental conception of normal regularity and order. It takes from the unfathomable depths of space and time those features of terror which an excited imagination is apt to ascribe to them. In all latitudes man, in the simple natural susceptibility of his mind, prizes “the calm stillness of a starlit summer
Although magnitude of space and mass appertains more especially to the sidereal portion of cosmical delineation, and the eye is the only organ of cosmical contemplation, our telluric sphere has, on the other hand, the preponderating advantage of presenting us with a greater and a scientifically distinguishable diversity in the numerous elementary bodies of which it is composed. All our senses bring us in contact with terrestrial nature, and while astronomy, which, as the knowledge of moving luminous celestial bodies is most acces